OK, I'm fed up with ATI not delivering any FreeBSD/Solaris drivers so I'm seriously considering moving to Nvidia. Before doing so, I would like to see the implications, though. Would I have to regenerate an xorg.conf file from scratch or can I keep the current ones (multiple distros) and simply use vesa during the transition? The file has quite a few "option" items which may not be applicable to Nvidia so I rather doubt that the latter would do. Or would it?

In my experience, you simply need to set the driver as "nvidia" and go. There are many different options that the Nvidia driver supports, but none of them appear required for operation.

as ms3gfx said, you just replace 'fglrx' or 'radeon', or whatever you have in the device section, with 'nv' or 'nvidia'. The nvidia driver is the closed source driver and has to be compiled first. The nv driver is open source and included with x.org, but non-3d (no gl, no xvideo and so on).
However, if you intend to buy one of those new gf8 cards (8800gts/x), you need at least driver version 9746 and the nv driver from x.org will not work. You can get a beta nv driver for gf8 from www.nvnews.net (go to the linux forum there).

The vesa driver should always work.

Thank you both for clearing that up. And i've always preferred the mid-range type of card so until nivia actually releases its 8600, I'll stick with the 7000 series.